Most companies, e.g. Family Tree DNA, simply use the SNPs from the chip used in the DNA test when reporting their results. The Autosomal SNP comparison chart at International Society of Genetic Genealogy (ISOGG) wiki states that Family Tree DNA test with 698,179 SNPs. They also report only using those SNPs.
GEDmatch's SNP counts are a bit higher than Family Tree DNA's because they include the extra SNPs from the raw data data supplied by Family Tree DNA and from other companies as well. The older chips from companies had mostly the same SNPs, and GEDmatch included the small number of extra SNPs from the other companies, whereas Family Tree DNA throws them away.
Lately, newer chips have been quite different, often not having enough overlap in their SNPs with older chips to allow them to easily compare the older with the newer. GEDmatch's solution to this was to create a separate area called GEDmatch Genesis for input of raw data from the new chips. That would include 23andMe v5 and Living DNA.
MyHeritage DNA does this differently. Their technique is to include every SNP from any company they accept input from. As described in the post MyHeritage DNA: Your Questions Answered
Q. How does MyHeritage match between users who tested on different services?
A. MyHeritage takes uploaded data and extrapolates the SNPs to a common ground. This is a process called imputation. Using this method MyHeritage can match any kit currently on the market or previously distributed including 23andMe’s V4 and Ancestry’s V2 chips (as well as earlier versions) and of course MyHeritage DNA. Imputation may introduce errors so we are in the process of fine-tuning it.
In other words, MyHeritage DNA will have many more SNPs between any two base pairs, because they will not only include SNPs from Family Tree DNA, but will also include the extra SNPs from the older chips like GEDmatch does, and they will also include all the different newer SNPs from the new chips that GEDmatch GENESIS also handle.
So comparing the number of SNPs is okay to do within a company, but the SNP counts are not comparable between companies. Family Tree DNA will have the lowest counts, GEDmatch will be next, and MyHeritage DNA will be much higher for the same segment.