I was wondering, given that a number represents a concept, unlike a letter that represents sound (that changes with context) - Do pictographic language users have similar rates of dyslexia and dysgraphia?
Short answer: Perhaps, but it is difficult to measure because the functional accounts of dyslexia and dysgraphia differ between alphabetic and non-alphabetic languages.
Longer answer: It has been argued that oral reading in Chinese can proceed via (at least) two pathways: the lexical semantic pathway (which supports reading for meaning), and the nonsemantic pathway (which connects representations of strokes, radicals, and characters to their phonological representations). 
From Yin et. al. :
"...The patterns of acquired dyslexia and dysgraphia observed in Chinese cannot be identical to the patterns observed in alphabetic languages. This is because certain properties of the Chinese script do not allow direct comparisons between the disorders. For example, nonlexical stimuli in alphabetic scripts...cannot be constructed in Chinese. Thus, it will not be possible to replicate the pattern of preserved nonword processing in surface dyslexia and dysgraphia observed in English and Spanish."
The article stresses that the patterns of dyslexia and dysgraphia in Chinese are determined by the language 'environment', or the spatial characteristics of the script, and not the language itself. Since the language is spatial in nature, it is hypothesized that speakers use orthographic information from the character in order to determine the character's meaning. This means that if access to the semantics of a word is partially damaged, then surface level dyslexia can be observed in Chinese -- though, again, since the nature of the languages is different, it is not an exact 'replica' of lexographic dyslexia in alphabetic languages.
Thus, it is more common in non-alphabet languages for dyslexia and dysgraphia to be associated with oral or phonological deficits. Some studies have shown that Chinese may demonstrate 'tonal dyslexia' by providing the wrong tonal stress on a syllable, or by providing the wrong monosyllable for the word.
"One view of phonological representations of Chinese words is that they have a non-linear structure containing separate syllabic, segmental (onset and rime) and supra-segmental layers...Tonal reading and writing errors result from impairment to the tonal tier or association between tonal and segmental tiers which leads to a dissociation between segmental and suprasegmental units, thus revealing the structure of the phonological lexicon."
Overall, given that there is no "universal" model of reading and writing, there is evidence that these dysfunctions in language processing arise within languages, not in spite of them, and thus take on different characteristics within each language. 
For individual characteristics among other non-alphabetic languages, feel free to look at the source material below.
Source used: Acquired Dyslexia and Dysgraphia Across Scripts
 To understand this within the context of alphetic languages, the dual-route theory of reading aloud describes observable potential deficits in lexical understanding and in phonological understanding when reading words aloud.
 W. Yin et al. / Acquired dyslexia and dysgraphia in Chinese (p. 163-165)
 B.S. Weekes / Acquired disorders of reading and writing: Cross-script comparisons (p. 53)